Conventionally, a so-called eccentrically oscillating gear system has been known which includes a ring-shaped internal gear having an inner circumferential surface including internal teeth, and an external gear placed inward of the internal gear to mesh with the internal gear while eccentrically oscillating, and including external teeth fewer in number than the internal teeth as described in, e.g., PATENT DOCUMENTS 1 and 2.
An input shaft of a gear system of PATENT DOCUMENT 1 includes an eccentric part, and the eccentric part is inserted through a bearing into a center hole formed in a central portion of an external gear so as to be rotatable about its axis. One end portion of a crankshaft is rotatably supported by a casing of the gear system, and the other end portion of the crankshaft is rotatably inserted into an insertion hole formed in a region of the external gear toward the outer circumferential surface thereof.
When, in the gear system, the input shaft is rotated with a motor, the movement of the eccentric part drives the external gear. The crankshaft is rotated with the external gear, and since the one end portion of the crankshaft is supported by the casing, the rotation of the external gear is blocked by the crankshaft, and the external gear is eccentric by the eccentricity of the eccentric part, and oscillates while meshing with the internal gear. Thus, the internal gear is rotated to output the torque of the motor.
A gear system of PATENT DOCUMENT 2 includes an external gear having a side surface on which many teeth are formed successively along the circumference of the gear. An input shaft includes, in addition to an eccentric part, an inclined shaft part extending obliquely relative to the center line of the eccentric part. A disk is rotatably supported by the inclined shaft part, and teeth meshing with the teeth on the side surface of the external gear are formed on a side surface of the disk.
In this gear system, rotation of the input shaft allows the external gear to move in response to the movement of the eccentric part. In this case, since the disk engages with the teeth on the side surface of the external gear, the external gear and the disk are connected together to allow the disk to block the rotation of the external gear. Thus, the external gear is eccentric by the eccentricity of the eccentric part, and oscillates while meshing with the internal gear.
The above-described eccentrically oscillating gear systems of PATENT DOCUMENTS 1 and 2 are compact in size, and can provide a high speed reducing ratio.